Dependence on energy and on potable water is an old concern of humanity. Energy and potable water demands have increased notoriously in the last few decades, and global warming is a problem that all nations understand must be addressed or future of humanity could be compromised.
Generating energy and producing potable water generally require burning hydrocarbons that contribute to the global warming problem. Until very recent times, coal, gas, petroleum, nuclear and hydropower have been the main sources of electric energy generation. Hydropower is a relatively small source of renewable energy.
Global warming, geo-politics and the possibility of exhausting the world's hydrocarbon deposits, have stimulated research on renewable energy sources. Both hydrothermal and wind are renewable sources of energy which have been used in the past; wind for several hundred years, and geothermal for about a century. For the past few decades several approaches to solar energy transformation to electricity have been used. In recent years great efforts have been made by scientists and engineers around the world to lower the technical and economic barriers to broaden use of winds, ocean waves, tides, solar radiation and the internal heat of the earth as alternative sources for renewable energy. However, there is a need for further improvements. The invention relates to a novel way of tapping the earth's internal heat energy to generate electricity while also providing potable water from the sea.
Humanity Urgently Needs Fresh Water and Electrical Energy.
There is a severe shortage of drinking water in many regions of the world and this situation will grow worse as population increases. Although the rate of population growth has been decreasing, it still remains high. By 2050 the global population is expected to be near nine thousand million (9 billion) Sachs [1].
The dry land surface of the earth has doubled since 1970. According to the Intergovernmental Panel on Climate Change, by 2050 this trend will worsen to the point of impacting food production. Growing one pound of wheat requires 520 liters of water, and growing a pound of rice requires 10,000 liters of water.
Underground water levels have decreased in many areas. For example, in some places bored wells that once hit water at 6 meters now must go down to about 24 meters. This problem is becoming more frequent around the world and is causing complicated migrations of people from dry lands.
At the present time about 350 million people live within 10 kilometers of a coast and about 3350 million, or approximately half of the world's population, live within 100 kilometers.
Scientists and engineers have tried to increase the availability of drinking water in different zones of the earth by modifying the local rain regime or by desalinating seawater. In the first case success has been relatively low, the latter has been done on a routine basis for a long time and its importance is growing.
There are three methods now in commercial use for sea water desalination. The first method employs evaporation and condensation of water in nuclear reactors. The second method employs evaporation and condensation of water heated using externally supplied electric power. In the third method, seawater is pressurized and passed through membranes which retain salts.
In all three methods the salts are extracted and returned to the sea or taken to special disposal areas on land. All of these methods are heavy consumers of energy supplied from external sources.
Desalination systems based on evaporation or osmosis techniques can be installed along most coastal regions of the world, provided that the electric power necessary for their operation is available.
In some regions of the world desalination is the main source of drinking water for millions of people, because there are few rivers and ground water is scarce or too deep. For instance, the Persian Gulf region has made significant investments in desalination systems and several are under construction to increase the supply of drinking water for its growing population. Something similar occurs in parts of the United States and other countries.
Some areas of the world are very dry due to low average annual rainfall, while other regions are very humid. Some areas that used to be fertile with abundant flora and fauna are now deserts. Rainfall, essential for life, is part of a larger phenomenon involving sea, air and the surface of the planet. This system is called the hydrologic cycle.
The sun's rays heat the sea surface and evaporate water. The water vapor moving by the action of the winds, change its temperature and pressure. Air masses ascend and cool forming droplets which fall as rain. Part of the water that falls evaporates; some infiltrates and some run off to the rivers that take it to the sea to continue the hydrologic cycle.
The hydrologic cycle involves complex physical phenomena where the atmospheric pressure and temperature, in addition to the global topography, are key variables. The sea temperature is also important and of lower relative importance is the surface temperature of the earth.
Temperature of the seawater varies from approximately 4° C. at the very deep ocean floor or polar seas, to 36° C. in some special areas of tropical seas. The seawater temperature depends largely on the broad range of factors from the atmosphere temperature and of the global ocean currents.
Energy supply is necessity for welfare of humankind. At the present time most energy consumed in the world comes from burning hydrocarbons. There is a growing participation of renewable energies, such as solar and wind sources. Hydropower and traditional geothermal generations are minor components in total energy annually produced. Huge annual investments are necessary to cope with the increasing demand for energy.
The Earth's Interior Thermal Energy
The earth's interior temperature varies from a few degrees ° C. on the surface to about 4200° C. at its center. Volcanoes are a demonstration of the earth's internal heat. The average surface temperature of the solid earth is about 10° C. and that of the sea floor is a little higher than 4° C.
The ratio of temperature to depth is the thermal internal gradient of the earth. It is expressed in degrees Celsius per kilometer (° C./km). The temperature rises rapidly with depth on the outside part of the earth's crust. This rate of increase declines at great depths, Press [2], Skinner et al [3]. The thermal gradient may vary from one site to another. Liyuang et al [4] have made careful measurements in a China drilling project where they found an average thermal gradient of 30° C./km for the first few kilometers of the earth's crust.
Analytical methods are used to estimate the earth interior temperature in terms of depth, varying some properties of the rock. Thermal gradients in different regions have been confirmed in oil drilling and in some special drillings made to investigate the earth's crust properties. The temperature change has also been confirmed in deep mines. Thermal flow is a slow process in the earth, due to the low thermal conductivity of rock.
The measurements allow deducing an average thermal gradient of 30° C./km for the first 10,000 meters to 20,000 meters of the earth's continental crust. The depth corresponds to measurements in land made from the medium sea level.
There is a very small heat loss due to radiation through the earth's surface. In spite of that radiation the earth's internal temperature remains almost constant. This is due to the heat liberated by radioactive materials such as thorium and uranium distributed in the interior of the planet. During the next million years the internal heat of the earth will remain more or less constant, Karato [5].
This virtually endless supply of internal heat combined with the huge amount of water in the oceans provides the potential for a new geothermal application to benefit humanity.
The Global Distribution of Water and Desalination
The earth's water is distributed approximately as follows: 97.3% in the oceans, 2.1% frozen at the poles, 0.6% in the more or less superficial groundwater, while lakes and rivers account for 0.2%. The atmospheric water vapor represents a very small proportion. It is estimated that the water of the planet corresponds to a volume of 1370 million cubic kilometers, USGS [6].
The minimum annual water consumption for an average person is the amount needed for drinking and daily needs, plus that necessary to produce a subsistence diet. This consumption level is close to one thousand cubic meters per year, Rogers [7]. By the year 2050 the planet's population could require about 9000 cubic kilometers of water for its subsistence. This is a negligible part of the total volume of available water on the earth.
The total amount of water on the planet and the annual consumption level for the human population show that the problem is not in the quantity of water but in its distribution in relation to the concentration of the population.
There are regions with abundant fresh water and others with chronic shortages that are increasing with time. There are regions with scarce fresh water but rapidly growing populations. Great efforts have been made in many coastal zones of the world to desalinize seawater as a solution to the shortages.
The salinity of water varies among different seas and even in the same sea. The salinity depends on local conditions such as temperature and water currents.
Desalination of seawater by evaporation in successive stages, and by passing it through membranes under pressure to retain the salt, allow for an economic large scale drinking water supply. These methods are the more favorable for marine water with variable salinity. They have been employed for decades and require large capital investments to construct the necessary facilities, and they incur ongoing expenses for large quantities of electric power.
Nuclear energy is also employed for desalination. In this method the desalinated water is a byproduct of electric power generation. New procedures in the future will reduce the cost of desalination by these methods.
In all the desalination processes, salts must be disposed of with minimum environmental impact. One possibility is to return the salts to the sea. Disposing of the salts may represent a significant cost in the desalination process, regardless of the desalination procedure
Besides the capital cost incurred in seawater desalination projects construction, electricity consumption represents an important component of the operating cost. The expense to generate electric power has both direct and indirect components. The direct cost is that to produce the energy itself. The indirect cost is that due to gases produced in coal or hydrocarbon burning by thermal plants, which contribute to global warming.
Desalination by evaporation in multiple stages (MSF) or by the multiple distillation method (MEF) require, on average, from 3 to 7.8 kilowatt-hours (kWh) to desalinize one cubic meter, Udono [8]. Most desalination systems consume electric power generated in plants that employ coal or fossil fuels as the thermal source.
Reduced environmental impact favors new seawater desalination procedures that do not use commercial electric power. Changing the proportion of energy production from traditional to alternative sources requires time and large financial efforts, National Petroleum Council [9].
Energy production in the world is distributed approximately as follows: 38% is obtained from oil, 23% is obtained from gas, 23% is obtained from coal, 7% is obtained in nuclear reactors, and 9% corresponds to renewable sources. Most scientists, engineers and leaders of the world today accept that there is a global warming phenomenon due, in part, to the burning of hydrocarbons to produce electricity.
Earth's Internal Temperature as a Source of Renewable Energy
At present very little of our renewable energy is made from geothermal sources produced by the internal heat of the planet. Traditional geothermal electric power generation is possible in regions with special thermal regimes, produced by intense activity of the internal forces of the earth near the surface. Electric power generation by means of traditional geothermal processes has the great advantage of its minor contribution to global warming.
In traditional geothermal electric power generation, water is forced to circulate to an appropriate depth and is vaporized or partially vaporized. The steam and hot water obtained are forced through a turbine and then are recycled. A production well and an injection well are necessary to obtain the heated fluid. About 30% of the pressurized injected water is recovered to produce electric power; this represents a relatively low efficiency. The traditional procedure does not desalinize water because it is restricted to very special zones of the planet.
Thousands of kilometers have been drilled by the oil industry. By the end of 2008 a well with depth of almost 10 kilometers was finished in the Gulf of Mexico. With the current drilling technology the use of geothermal energy based in very deep drilling becomes a real possibility. In the traditional geothermal electric power generation the maximum depth wells do not surpass a few thousands of meters, but in order to use what is called dry rock geothermal energy, efforts are being made to go down to ten thousand meters. With depth increasing, the drilling becomes very complex due to high temperatures and pressures and to the large diameter of the well.
To heat up water inside the dry rock, the thermal energy transfer must be stimulated with the presence of enough natural fractures or porosity in the rocky medium. If this is not the case, artificial methods like fracturing the rock can be used to circulate injected water to some volume around the geothermal region employed to heat water to produce electricity [10].